Since at least the early 1930's, newly manufactured automobiles have been equipped with hydraulically operated brakes. Typically, hydraulic brake fluid is maintained in one or more reservoirs in a master cylinder which is actuated by a foot operated brake pedal in the vehicle operator's cab. Hydraulic fluid pressure is applied through hydraulic lines to actuating wheel cylinders located on each of the wheels of the vehicle. The wheel cylinders are pressurized to operate either a brake shoe (or shoes) against a brake drum or a caliper assembly which operates brake pads against a metal disk to decelerate or stop the vehicle.
It is well known that when such a system is charged with hydraulic fluid, or whenever air is allowed to enter the system for any reason, it is very important to expel all of the air in the system before sealing it for use. If this is not done properly, the air left in the system will be compressed by pressing the brake pedal and the pedal will feel "soft." Worse, where there is more than a minimal amount of air in the system, the brake pedal may go so low as to prevent proper application of the brakes and create a serious safety hazard.
It will be well understood by one of ordinary skill in this art that the same principle would apply to nearly any other hydraulic system employing an actuating cylinder and a pressure producing or operating cylinder. In any such system, it is important to prevent gas (air) entry into the system and to fully expel any such gas in a purging or bleeding process.
There are two well known methods of bleeding the air or other gas out of such a system:
Where the proper equipment is available, bleeding has been accomplished by applying fluid pressure continuously from the master cylinder by applying air pressure at the top of the reservoir and, one by one, or in concert, opening bleed valves which are typically mounted at each wheel cylinder. Air and hydraulic brake fluid is expelled at the opened bleed valve(s) until a steady flow of fluid is emitted without any air content, at which point the bleed valve(s) is closed.
The second method, used where no special equipment is available, comprises depressing the brake pedal, thereby applying fluid pressure to the hydraulic lines, opening a bleed valve (a part of typical prior art brake system wheel cylinders), usually at a single wheel, closing the bleed valve before the brake pedal is released, releasing the brake pedal. This procedure is then repeated until no air bubbles emerge with the fluid at the bleed valve. The procedure is repeated for each wheel until all lines are bled.
In each of these procedures, it is common practice to discard the hydraulic fluid which is expelled from the wheel cylinder bleed valves to prevent contamination of a system which could result from the use of dirty recycled fluid. Such contamination is generally believed to result from exposure of fluid to a dirty atmosphere.
A drawback of each of the systems described is the difficulty in making a determination of exactly when the bleeding process has been completed since an operator must make a decision, while lying prone or standing in a pit beneath the vehicle, as to whether or not there are any air bubbles in the fluid emitted from the bleed valve(s). This is usually accomplished by attaching a short flexible line to each bleed valve as it is used and leading that line below a small amount of hydraulic fluid in a transparent vessel. This facilitates watching for air bubbles. Not infrequently, the collecting vessel is overturned, broken or the short tubing slips out of the vessel (the hydraulic fluid wetted tubing is very slippery) and fluid is spilled beneath the vehicle. It is also possible for an inattentive operator to allow the collecting vessel to fill to overflowing with the same result.
Each of these bleeding methods is labor intensive in that one or more operators must be employed to assure that no new air is introduced into the hydraulic lines and that the master cylinder reservoir does not run dry thereby introducing new air into the system. It is necessary to close each bleed valve while pressure is still being applied at the master cylinder to assure that no air is taken into the system from the bleed valve at each wheel. If multiple lines are bled simultaneously, good procedure demands that an operator be stationed at each wheel cylinder to control that cylinder's bleed valve at the proper time.